HTMLSelectElement: value 属性

<label for="bird-select">Choose a bird:</label>

<select name="birds" id="bird-select">
  <option value="">--Please choose an option--</option>
  <option value="Scarlet ibis">Scarlet ibis</option>
  <option value="Marabou stork">Marabou stork</option>
  <option value="Roseate spoonbill">Roseate spoonbill</option>
</select>

<pre id="log"></pre>

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = text;
}

const select = document.querySelector("#bird-select");
select.addEventListener("change", () => {
  log(`Selection: ${select.value}`);
});

#log {
  height: 20px;
  padding: 0.5rem;
  border: 1px solid black;
}